A self-powered central heating unit is one that operates entirely on combustion fuel, independently from the electrical grid. Such a system would in theory be more reliable in providing heat through extreme weather conditions. It would also find application in remote habitations not served by the electrical grid. However, the main obstacle to integrating an electric generator into a heating unit is the selection of a convenient, cost-effective engine and generator. Self-powered central space heating with forced air has been demonstrated utilizing expensive lead-telluride based thermoelectric devices. However, these devices have not been commercially successful. These devices are very inefficient and expensive and operate at a relatively low maximum hot side temperature of about 500 degrees F.
Thermoelectric devices are well known. These devices utilize physics principale known as the Seebeck effect discovered in 1821 and the Peltier effect discovered in 1834. The Seebeck principle tells us that if two wires of different materials (such as copper and iron) are joined at their ends, forming two junctions, and one junction is held at a higher temperature than the other junction, a voltage difference will arise between the two junctions. The Peltier effect describes an inverse effect. If current is sent through a circuit made of dissimilar materials, heat will be absorbed at one junction and given up or evolved at the other junction.
Most thermoelectric devices currently in use today to generate electricity or for cooling utilize semiconductor materials (such as bismuth telluride) which are good conductors of electricity but poor conductors of heat. These semiconductors are typically heavily doped to create an excess of electrons (n-type) or a deficiency of electrons (p-type). An n-type semiconductor will develop a negative charge on the cold side and a p-type semiconductor will develop a positive charge on the cold side.
Since each element of a semiconductor thermoelectric device will produce only a few millivolts it is generally useful to arrange the elements in series so as to produce higher voltages for the generation of electricity or to permit use of higher voltages for cooling. Several techniques have been developed for arranging the semiconductor elements in series in thermoelectric devices. In one such prior art method p and n type semiconductors are arranged in a checkerboard pattern and electrical jumpers are soldered, each to two different semiconductors, at the cold side and at the hot side so as to place all of the semiconductor elements in series with each other. This method is a low cost method and well established but has some limitations. Above 100 C the solders can defuse into the thermoelectric elements destroying them. In a high humidity atmosphere moisture may condense in the spaces between the elements and thermally short the module. The structure is not mechanically strong and easily fractures.
Another currently used method is the so-called eggcrate design. Here an "eggcrate" made of insulator material separates the thermoelectric elements and permits electrical jumpers to be pressed against the elements to provide a good electrical connection without solder. In prior art designs, the eggcrates are fabricated from individual walls which have been cut to shape using a precision laser cutter. All of the elements can be connected in series by proper construction of the eggcrate. Obviously it is possible in both devices to arrange for any desired number of elements to be in series. Thus, several elements in series may form a series set and this set could be arranged in parallel with other similar sets.
Prior art thermoelectric generators are expensive due primarily to labor costs to assemble the eggcrates to install the elements in the crates and to form the modules into a generator unit. What is needed is a lower cost thermoelectric generator.
What is needed is a better more efficient, cheaper, higher temperature self-powered central home heating unit.